qapi: Catch and reject flat union branch of array type

Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

scripts/qapi.py

@@ -580,7 +580,7 @@ def check_union(expr, expr_info):
         # Each value must name a known type; furthermore, in flat unions,
         # branches must be a struct with no overlapping member names
         check_type(expr_info, "Member '%s' of union '%s'" % (key, name),
-                   value, allow_array=True, allow_metas=allow_metas)
+                   value, allow_array=not base, allow_metas=allow_metas)
         if base:
             branch_struct = find_struct(value)
             assert branch_struct

tests/qapi-schema/flat-union-array-branch.err

-Traceback (most recent call last):
-  File "tests/qapi-schema/test-qapi.py", line 19, in <module>
-    exprs = parse_schema(sys.argv[1])
-  File "scripts/qapi.py", line 760, in parse_schema
-    return check_exprs(schema.exprs)
-  File "scripts/qapi.py", line 743, in check_exprs
-    check_union(expr, info)
-  File "scripts/qapi.py", line 586, in check_union
-    assert branch_struct
-AssertionError
+tests/qapi-schema/flat-union-array-branch.json:8: Member 'value1' of union 'TestUnion' cannot be an array